Pneumatic suspension for a motorcar

ABSTRACT

A pneumatic suspension for a motorcar has a level indicator switch having a high load position and a low load position in which it is connected by first and second switches with an electric motor-compressor, and an electromagnetic relief valve, respectively. A third switch connects the motor-compressor parallel to the level indicator switch into the circuit. The three switches are part of a control switch means, and are operated by a spring biassed pressure responsive element having a piston portion in a pressure chamber communicating with the pneumatic cylinder and piston means by which the chassis is supported on a wheel shaft. By use of a strong spring for the pressure responsive element, the operation of the first switch takes place only at a predetermined high load.

United States Patent [1 1 Engfer PNEUMATIC SUSPENSION FOR A MOTORCAR[75] Inventor: Ortwin Engfer, Gerlingen, Germany [73] Assignee: RobertBosch GmbH, Stuttgart, Germany [22] Filed: Mar. 12, 1971 [21] Appl. No.:123,528

[l1] 3,729,21 1 45] Apr. 24, 1973 Att0rney-Michael S. Striker 57ABSTRACT A pneumatic suspension for a motorcar has a level in dicatorswitch having a high load position and a low load position in which itis connected by first and second switches with an electricmotor-compressor, and an electromagnetic relief valve, respectively. Athird switch connects the motor-compressor parallel to the levelindicator switch into the circuit. The three switches are part of acontrol switch means, and are operated by a spring biasscd pressureresponsive element having a piston portion in a pressure chambercommunicating with the pneumatic cylinder and piston means by which thechassis is supported on a wheel shaft. By use of a strong spring for thepressure responsive element, the operation of the first switch takesplace only at a predetermined high load.

9 Claims, 4 Drawing Figures Patented April 24, 1973 3 Sheets-Sheet 1 Im'vsurok din/m E06F lid 10 I 0! I) Patentd April 24, 1973 I 3,729,211

3 Sheets-Sheet 2 Fig. 2

(at/m 0 '1 v '2 30in) INVENTOR.

agruw E06 Fell BY Patented April 24, 1973 3,729,211

3 Sheets-Sheet 5 INVENTOR.

OIZI'UIM EH6 FEE ll/0r?! I PNEUMATIC SUSPENSION FOR A MOTORCARBACKGROUND OF THE INVENTION The present invention relates to a pneumaticsuspension for supporting a chassis on a wheel shaft. Pneumaticsuspensions of this type are known which include an electric levelindicator switch responsive to relative motion between the chassis andthe wheel shaft, a motor compressor controlled by a relay, and anelectromagnetic relief valve for relieving the cylinder and piston meansby which the chassis is supported on the wheel shaft.

Elastic pneumatic suspensions according to the prior art have thedisadvantage that an additional load acts not only on the controlledwheel shaft, but also on the other wheel shaft of the motorcar, so thatan additional load may cause a slanted position of the chassis, whichhas a detrimental influence on the direction of the beam of theheadlights.

It is one object of the invention to overcome this disadvantage, and toprovide a pneumatic suspension arrangement in which the level regulationtakes place only at a predetermined high load, for example, if more thantwo persons enter the motorcar.

Another object of the invention is to facilitate the changing of tires,and the raising of the motorcar for servicing operations withoutdisconnection of the elastic pneumatic suspension, and without anydanger for the battery.

Another object of the invention is to protect the cylinder and pistonmeans of the pneumatic suspension against extremely low pressure, and toprotect the motorcar from overloads.

With these objects in view, the present invention provides a pressureresponsive control switch means with three switches which, depending onthe pressure in the pneumatic suspension, effect in one of two pressurestages, respectively, a pressure variation, and in the third pressurestage adjustment of extremely low pressure.

An embodiment of the invention comprises a wheel shaft; a chassis;pneumatic cylinder and piston suspension means for mounting the chassison the wheel shaft; an electric motor compressor, preferably including arelay, and an electromagnetic relief valve communicating with a cylinderand piston means for raising and lowering the chassis, respectively;level indicator switch means moving to a first'position when the chassisis too low and to a second position when the chassis is to high; andpressure responsive control switch means including a pressure chambercommunicating with the cylinder and piston means, a pressure responsiveelement in the pressure chamber, and first, second, and third switchescontrolled by the pressure responsive element dependent on the pressurein the cylinder and piston means and on the corresponding pressure inthe pressure chamber.

In accordance with the invention, the first switch connects the levelindicator switch means in the first position with the relay of the motorcompressor, and the second switch connects the level indicator in thesecond position with the electromagnetic relief valve. Consequently, themotor compressor is started when the load on the chassis is increased,and the relief valve is opened when the load on the chassis is reduced.The third switch effects energizing of the motor compressor when thepressure in the cylinder and piston means and the pressure chamber istoo low, and is connected in parallel with the level indicator switchmeans.

The relay of the motor compressor and the electromagnetic relief valveare connected in parallel to a voltage source, and the movable switcharm of the level indicator switch means is connected to ground, and bythe first and second switches with the motor compressor and reliefvalve. The third switch directly connects the relay of the motorcompressor with ground.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an axial sectional viewillustrating a pressure responsive control switch according to apreferred embodiment of the invention;

FIG. 2 is a diagram illustrating the variation of the pressure over thedistance of travel of a pressure responsive element of the controlswitch;

FIG. 3 is a circuit diagram of the control circuit of the pneumaticsuspension shown in FIG. 4; and

FIG. 4 is a schematic view ofa pneumatic suspension controlled inaccordance with the invention, including air conduits, and electricconnections.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings,and particularly to FIG. 1 which illustrates a pressure responsivecontrol switch 1 according to the invention, a stepped cylindricalhousing 2 has a cylindrical pressure chamber 9 bounded by a cylindricalsurface 3 of housing 2, and by a closure member 4 which is secured tothe rim of housing 2 and sealed by a sealing ring. The bottom closure 4has a connector 5 for a pressure conduit 6 communicating with thecylinders of pneumatic cylinder and piston means 41 and 42 which connecta wheel shaft 37 with wheels 38 and 39 with a chassis 40, as best seenin FIG. 4.

The tubular connector 5 has a bore 7 into which the stem of a throttlepin 8 is inserted whose head projects into the pressure chamber 9, andis biassed by a spring 10 to press the head of throttle pin 8 against anend face 11 of the end portion 14 of a stem 17 which has a steppedflange 15 abutting an elastic piston 12 which is secured by a ring tothe end portion 14, and moves with the stem 17, 15, so that the element14, 15, 17 and 12 constitute a pressure responsive element responding tothe pressure in chamber 9, which corresponds to the pressure in thepneumatic suspension cylinder and piston means 41 and 42.

Pneumatic suspension cylinder andpiston means 41 and 42 communicate withpressure chamber 9 through a throttling gap 13 between throttle pin 8and the inner surface of connector 7, so that the pressure in pressurechamber 9 is reduced.

The end face 11 on which the head of throttle pin 8 abuts due to theaction of spring 10, is the bottom of a recess in the end portion 14which is bounded by acircular wall having bores communicating withpressure chamber 9. "a i A I J I The stepped housing has an innerannular shoulder 2' on which thelarg er portion of flange 15 abuts whena predetermined maximum pressure in pressure chamber 9 is reached, andthe membrane piston portion 12 is correspondingly deformed.

A weak coil spring 19 abuts at one end the upper portion 18 of thehousing, and at the other end flange 15 so that piston 12 is urged to anormal position. A strong spring 20 has ends abutting the top portion 18of the housing, and a ring 21 which abuts a spring ring 22 inserted intoa groove in the housing so that spring 20 is confined in the housingportion 16, without normally influencing flange 15 from which ring 22 isspaced a small axial distance.

However, if the pressure in pressure chamber 9 increases, and piston 12is pressed upward so that the pressure responsive element 17,14, 12, 15is displaced, this small diameter portion of flange 15 will engage ring22 and compress the strong spring 20 before the large diameter portionof flange l abuts the annular abutment shoulder 2 of the housing.

Movement of flange 15 into engagement with ring 21 will require onlysmall pressure for overcoming the.

force of the weak spring 19, but further movement of flange 15,requiring compression of the strong spring 20, will require a fargreater pressure in the pressure chamber 9, and in the pneumaticsuspension.

The upper end of the stem 17 forms an actuating portion projecting outof the tubular portion 24 of the top wall 18 of housing 2. A tubularbushing 25 is mounted on portion 24, and supports three switches 26, 27,28 in parallel positions and staggered relative to each other to thedirection of movement of stem 17 and its actuating end face 23 on whichthe actuators 26', 27, 28 abut.

The first, second, and third switches 28, 27, 26 are staggered distancesof about 2 mm, so that the actuating portions 28', 27', 26 projectdifferent distances out of the respective spring housings intoengagement with the actuating end face 23.

FIG. 1 shows the control switch 1 in a normal intermediate position inwhich the lower end 14 of stem portion 14 has a distance of 1.5 mm fromthe bottom closure 4, while the top face of flange 15 has a distance of1.5 mm from the abutment shoulder 2 of housing 2. Consequently, the stem17, 14 can move with its flange 15 with 1.5 mm, and downward also 1.5mm, and has a complete stroke of 3 mm.

Referring now to FIG. 4, the pneumatic cylinder and piston means 41, 42connect a chassis 40 with the wheel shaft 37 carrying two wheels 38 and39. A level indicator switch has a switch arm 34 mounted on a fulcrum onchassis 40 for angular movement, and is connected by a linkage 43 to theshaft 37. Switch arm 34 has two positions in which switches 36 and 35are connected to ground, as schematically shown in FIG. 3. 1

The air conduit 6 connects the cylinders of the pneumatic suspension 41and 42 with the pressure chamber 9 of control switch 1, as explainedwith reference to FIG. 1, and also with a compressor 37 driven by anelectric motor 46, and pumping air from an air filter 44 into thecylinders of the pneumatic suspension means 41 and 42, if motor 46 isenergized. The motor 46 is connectedin series with the contact of'arelay 31 which is connected with two parallel switches 26, 28 of thecontrol switch 1, as best seen in FIG. 3. The air conduit 6 is alsoconnected to a relief valve 33 which has a normal position shown in FIG.4, in which relief valve 33 is closed, and an actuated position in whichthe air conduit 6 is connected with a return conduit to the inlet ofcompressor 37. The relief valve 33 is shifted by electromagnetic means,schematically shown in FIG. 4 to be connected by a main switch 45 to avoltage, and being also connected with switch 27 of control switch means1, as best seen in FIG. 3.

FIG. 3 shows only compressor 37, and it will be understood that itsmotor 46 is controlled by the relay 31. The winding of relay 31 isconnected in series with switch 28, contact 35, and switch arm 34 toground. Relay 31 is also connected through switch 26 with ground. Theelectromagnet of relief valve 33 is connected to switch 44, and inseries with switch 27 of the control switch means 1, as schematicallyindicated by a rectangle appearing in chain lines. Relief valve 33 andswitch 27 re connected through contact 36 and switch arm 34 to ground.

Switches 26 and 28 are normally closed, and switch 27 is normally open,and these switches are opened and closed by the movement of the pressureresponsive element 17, 15, 14, 12, as described with reference to FIG.1.

If the pressure in the suspension cylinders 41, 42 has droppedconsiderably, for example to 0.5 atm., so that the end 14 of stem 14abuts bottom closure 4 due to the action of the weak spring 19, theswitch 26 is closed, and a current flows through the relay 31 and switch26 to ground, entirely independently of the position of the levelindicator switch means 34. Relay 31 starts motor 46 of compressor 37which pumps air into the cylinders of the pneumatic suspension 41, 42 sothat the pressure in the same rises, resulting in a correspondingpressure rise in the pressure chamber 9 of control switch 1 until at thepressure of 1 atm., the pressure responsive element in control switchmeans 1 again assumes its normal position, so that switch 26 is opened,and the compressor 37 stops. Due to the provision of switch 26, thepressure in the pneumatic suspension 41, 42 is always maintained at thedesired minimum level so that damage to the cylinders and pistons of thepneumatic suspension is prevented.

The function of switch 26 is particularly necessary if the pneumaticsuspension include flexible bellows.

The pressure function during this operation is shown in FIG. 2 betweenthe points 26E and 26A, the character E representing start of thecompressor, and the character A representing stopping of the compressor.It will be seen that the pressure responsive element moves less than 1mm to maintain the pressure at 1 atm. I

Switch 27 is operated by the pressure responsive element when the loadon chassis 40 is reduced, so that the same rises and actuates the levelindicator switch 34 to connect the electromagnetic valve 33 throughswitch 27 and contact 36 to ground so that a current flows through theelectromagnetic relief valve 33 and the same opens until the existingpressure in the pneumatic suspension4l has dropped to 1 atm. When thechassis 40 rises due to the reduced load, there is still comparativelyhigh pressure in pressure chamber 9 and switch 27 is closed. When therelief valve 33 is operated to reduce the pressure to 1 atm., thepressure responsive element moves to its normal position opening switch27 so that relief valve 33 closes.

Upon a small load increase, chassis 40 moves downward and levelindicator switch 35 closes. The circuit of compressor 37 is closed. Thecompressor 37 supplies the pneumatic suspension cylinders 41, 42, untilthe chassis reaches its original level. Then level indicator switch 35opens and compressor 37 is stopped.

Upon a great load increase chassis 40 moves also downward and levelindicator switch 35 closes. Now compressor 37 supplies the pneumaticcylinders 41, 42 for a long periode and stops only, if the chassisreaches its original level. Should the chassis however not lift, thehigh pressure prevails in the pressure chamber 19 of the controll switch1, so that flange abuts abutment shoulder 2' and also engages ring 21for compressing the strong spring a small distance. During this fillingperiod, the pressure follows the vertical line in FIG. 2, since a greatcounterforce exerted by the strong spring 20 has to be overcome. Whenthe maximum pressure is reached, the switch 28 disconnects the fillingof the cylinders of the pneumatic suspension 41,42, whereupon thecircuit of the relay 3] of compressor 37 is interrupted.

FIG. 2 shows the steep increase of the pressure, and the regulationbetween the points 28E and 28A.

The throttling duct 13 serves the purpose to prevent response of thepressure responsive element of control switch means 1 to dynamicvariations and fluctuations in the hydraulic system. Since thethrottling duct is very narrow, it is necessary to provide axialmovability of throttle pin 8, so that the annular throttling duct 13 iscontinuously cleaned.

As can be seen from FIGS. 3 and 4, the arrangement of the inventionpermits the changing of tires, and the raising of the motorcar bylifting apparatus to a servicing position in which it is accessible frombelow. The chassis is raised, so that its distance from the wheel shaft37 is increased. The level indicator switch means 34 assumes itsposition for a reduced load, so that contact 36 is connected to ground,and the relief valve 33 is energized to relief the pressure in thepneumatic suspension. When at the point 27A in FIG. 2, a pressure of 1atm. is reached, switch 27 opens, and the relief valve 33 closes so thata pressure of 1 atm. is maintained in the system.

Due to this arrangement, it is not necessary to disconnect the pneumaticsuspension when a tire is changed, or the car is raised. Since theelectromagnetic relief valve 33 is not normally energized, no current isconsumed, and the battery does not have to supply a currentcontinuously, and is consequently protected from being completelydischarged.

It will be understood that each of the elements described above, or two.or more together, may also find a useful application in other types ofpneumatic suspensions differing from the types described above.

While the invention has been illustrated and described as embodied in apneumatic suspension with a pressure responsive control switchelectrically operating a compressor and a relief valve, it is notintended to be limited to the details shown, since various modificationsand structural changes may be made without departing in any way from thespirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

1 claim:

1. Pneumatic suspension for a motorcar, comprising a wheel shaft; achassis; pneumatic cylinder and piston means for mounting said chassison said wheel shaft; an electric motor compressor and an electromagneticrelief valve communicating with said cylinder and piston means forraising and lowering said chassis, respectively; level indicator switchmeans responsive to the relative position between said chassis and saidwheel shaft to move to a first low-load position when said chassis istoo low and to a second high-load position when said chassis is toohigh; and pressure responsive control switch means including a housingforming at one end a cylindrical pressure chamber communicating withsaid cylinder and piston means, first, second, and third switchesmounted at the other end of said housing adjacent each other, a pressureresponsive element having at one end a piston portion located in saidcylindrical pressure chamber, and having at the other end an actuatingportion for operating said first, second and third switches dependent onthe pressure in said cylindrical pressure chamber and in said cylinderand piston means, and biassing means in said housing biassing saidpressure responsive element against the pressure in said pressurechamber, said first switch connecting said level indicator switch meansin said first position with said motor-compressor, and said secondswitch connecting said level indicator means in said second positionwith said electromagnetic relief valve so that said motor-compressor isstarted when the load on said chassis is increased, and said reliefvalve is opened when the load on said chassis is reduced, and said thirdswitch effecting energizing of said motor-compressor when the pressurein said cylinder and piston means and in said pressure chamber is toolow, and being connected in parallel with said level indicator switchmeans.

2. Pneumatic suspension as claimed in claim 1 wherein said switches aredisposed in said housing staggered in the direction of movement of saidpressure responsive element so that said first, second, and thirdswitches are operated in different positions of said pressure responsiveelement depending on the pressure in said pressure chamber and on theforce of said spring means.

3. Pneumatic suspension as claimed in claim 2 wherein said switches areparallel, and wherein said actuating portion of said pressure responsiveelement abuts said switches.

4. Pneumatic suspension as claimed in claim 2 wherein said spring meansinclude a weak spring having one end abutting said housing and saidpressure responsive element for urging the same in one direction againstthe pressure in said pressure chamber to a normal position if' apredetermined normal pressure prevails in said pressure chamber, andbeyond said normal position to a low pressure position if the pressurein said pressure chamber and in said cylinder and piston means dropsbelow said normal pressure, and a strong spring abutting at both ends onportions of said housing, and being engaged by said pressure responsiveelement when the same moves in the opposite direction out of said normalposition due to increased pressure in said pressure chamber so that asubstantial increase of the pressure in said cylinder and piston meansand of the load acting on said chassis is required for moving saidpressure responsive element in said opposite direction against theaction of both said weak and strong springs.

5. Pneumatic suspension as claimed in claim 4 wherein said pressureresponsive element operate said third switch in said low pressureposition so that said motor compressor is energized, and again in saidnormal position so that said motor compressor is deneergized at normalpressure in said cylinder and piston means, and in said pressurechamber.

6. Pneumatic suspension as claimed in claim 4 wherein said pressureresponsive element operates said first switch to energize saidmotor-compressor when the load on said chassis is increased apredetermined amount, and thereby the pressure in said pressure chamberis increased to a predetermined pressure sufficient to move saidpressure responsive element in said opposite direction against theaction of said strong spring.

7. Pneumatic suspension as claimed in claim 4 wherein said pressureresponsive element operates said second switch when the load on saidchassis is reduced so that said relief valve is opened whereby thepressure in said cylinder and piston means is reduced to said normalpressure, whereupon said second switch is again operated by saidpressure responsive element to close said relief valve.

8. Pneumatic suspension as claimed in claim 4 wherein said pressureresponsive element operates said first switch to energize saidmotor-compressor when the load on said chassis is increased apredetermined amount, and thereby the pressure in said pressure chamberis increased to a predetermined pressure sufficient to move saidpressure responsive element in said opposite direction against theaction of said strong spring; wherein said pressure responsive elementoperates said second switch when-the load on said chassis is reduced sothat first said relief valve is opened whereby the pressure in saidcylinder and piston means is reduced to said normal pressure, whereuponsaid second switch is again operated by said pressure responsive elementto close said relief valve; wherein said pressure responsive elementoperates said third switch in said low pressure position so that saidmotor compressor is energized, and again in said normal position so thatsaid motor compressor is deenergized at normal pressure in said cylinderand piston means, and

in said ressure chamber.

neumatlc suspension as claimed in claim 8 wherein said pressureresponsive element includes a stem having said actuating portion at saidother end, a flexible membrane forming said piston portion mounted atsaid one end, and a flange on said one end abutting said membrane; andwherein said housing has an annular shoulder on which said flange abutsin an end position of said pressure responsive member when said weak andstrong springs are deformed by said pressure responsive element to apredetermined degree at a predetermined high load.

1. Pneumatic suspension for a motorcar, comprising a wheel shaft; achassis; pneumatic cylinder and piston means for mounting said chassison said wheel shaft; an electric motor compressor and an electromagneticrelief valve communicating with said cylinder and piston means forraising and lowering said chassis, respectively; level indicator switchmeans responsive to the relative position between said chassis and saidwheel shaft to move to a first low-load position when said chassis istoo low and to a second high-load position when said chassis is toohigh; and pressure responsive control switch means including a housingforming at one end a cylindrical pressure chamber communicating withsaid cylinder and piston means, first, second, and third switchesmounted at the other end of said housing adjacent each other, a pressureresponsive element having at one end a piston portion located in saidcylindrical pressure chamber, and having at the other end an actuatingportion for operating said first, second and third switches dependent onthe pressure in said cylindrical pressure chamber and in said cylinderand piston means, and biassing means in said housing biassing saidpressure responsive element against the pressure in said pressurechamber, said first switch connecting said level indicator switch meansin said first position with said motor-compressor, and said secondswitch connecting said level indicator means in said second Positionwith said electromagnetic relief valve so that said motor-compressor isstarted when the load on said chassis is increased, and said reliefvalve is opened when the load on said chassis is reduced, and said thirdswitch effecting energizing of said motor-compressor when the pressurein said cylinder and piston means and in said pressure chamber is toolow, and being connected in parallel with said level indicator switchmeans.
 2. Pneumatic suspension as claimed in claim 1 wherein saidswitches are disposed in said housing staggered in the direction ofmovement of said pressure responsive element so that said first, second,and third switches are operated in different positions of said pressureresponsive element depending on the pressure in said pressure chamberand on the force of said spring means.
 3. Pneumatic suspension asclaimed in claim 2 wherein said switches are parallel, and wherein saidactuating portion of said pressure responsive element abuts saidswitches.
 4. Pneumatic suspension as claimed in claim 2 wherein saidspring means include a weak spring having one end abutting said housingand said pressure responsive element for urging the same in onedirection against the pressure in said pressure chamber to a normalposition if a predetermined normal pressure prevails in said pressurechamber, and beyond said normal position to a low pressure position ifthe pressure in said pressure chamber and in said cylinder and pistonmeans drops below said normal pressure, and a strong spring abutting atboth ends on portions of said housing, and being engaged by saidpressure responsive element when the same moves in the oppositedirection out of said normal position due to increased pressure in saidpressure chamber so that a substantial increase of the pressure in saidcylinder and piston means and of the load acting on said chassis isrequired for moving said pressure responsive element in said oppositedirection against the action of both said weak and strong springs. 5.Pneumatic suspension as claimed in claim 4 wherein said pressureresponsive element operate said third switch in said low pressureposition so that said motor compressor is energized, and again in saidnormal position so that said motor compressor is deneergized at normalpressure in said cylinder and piston means, and in said pressurechamber.
 6. Pneumatic suspension as claimed in claim 4 wherein saidpressure responsive element operates said first switch to energize saidmotor-compressor when the load on said chassis is increased apredetermined amount, and thereby the pressure in said pressure chamberis increased to a predetermined pressure sufficient to move saidpressure responsive element in said opposite direction against theaction of said strong spring.
 7. Pneumatic suspension as claimed inclaim 4 wherein said pressure responsive element operates said secondswitch when the load on said chassis is reduced so that said reliefvalve is opened whereby the pressure in said cylinder and piston meansis reduced to said normal pressure, whereupon said second switch isagain operated by said pressure responsive element to close said reliefvalve.
 8. Pneumatic suspension as claimed in claim 4 wherein saidpressure responsive element operates said first switch to energize saidmotor-compressor when the load on said chassis is increased apredetermined amount, and thereby the pressure in said pressure chamberis increased to a predetermined pressure sufficient to move saidpressure responsive element in said opposite direction against theaction of said strong spring; wherein said pressure responsive elementoperates said second switch when the load on said chassis is reduced sothat first said relief valve is opened whereby the pressure in saidcylinder and piston means is reduced to said normal pressure, whereuponsaid second switch is again operated by said pressure responsive elementto close said relief valve; wherein said pressure responsive elementopErates said third switch in said low pressure position so that saidmotor compressor is energized, and again in said normal position so thatsaid motor compressor is deenergized at normal pressure in said cylinderand piston means, and in said pressure chamber.
 9. Pneumatic suspensionas claimed in claim 8 wherein said pressure responsive element includesa stem having said actuating portion at said other end, a flexiblemembrane forming said piston portion mounted at said one end, and aflange on said one end abutting said membrane; and wherein said housinghas an annular shoulder on which said flange abuts in an end position ofsaid pressure responsive member when said weak and strong springs aredeformed by said pressure responsive element to a predetermined degreeat a predetermined high load.